The present invention relates to a method and an apparatus for treatment of exhaust gas having been generated in manufacturing processes for semiconductor devices, and more particularly to a method and an apparatus for treatment of unreacted PFC gas to avoid direct discharge of the unreacted PFC gas to an atmosphere.
In the semiconductor device manufacturing processes, a PFC gas or a freon gas comprising carbon and fluorine has been used. The PFC gas has been used, for example, as an etching gas in the dry etching process or a cleaning gas in the plasma CVD apparatus. The PFC gas is much higher in global warming power coefficient (GWP) by about a few thousands than that or carbon dioxide, for which reason if the used PFC gas is discharged directly to atmosphere, then a certain influence on the global atmospheric system is concerned. It has been required to control the direct discharge of the used PFC gas to the atmosphere.
In the conventional dry etching apparatus, PFC gases such as CF.sub.4, C.sub.2 F.sub.6 are used as etching gases. As illustrated in FIG. 1, a PFC gas is introduced as a source gas into a dry etching apparatus 1. PFC gas is then decomposed by a radio frequency plasma whereby not only etching species or etchants but also unreacted PFC gas and etching reaction products are generated. The etching species or etchants are used for the dry etching process. On the other hand, the unreacted PFC gas and reaction products are discharged by a pump 2 from a reaction chamber in the dry etching apparatus 1. The discharged unreacted PFC gas and etching reaction products are fed to a removal apparatus 3 for removing the reaction products.
If the C.sub.2 F.sub.6 gas is used as an etching gas for etching a silicon oxide film, then etching reaction gases such as SiF.sub.4, COF.sub.2 are generated. In order to remove such undesired etching reaction gases, it is effective to use an adsorption removal method by use of adsorbents such as zeolite or activated carbon for removal thereof. Since, however, the unreacted PFC gas is chemically stable, the unreacted PFC gas is not adsorbed onto the adsorbents such as zeolite or activated carbon, for which reason conventionally the unreacted PFC gas is obliged to be discharged directly.
The plasma etching apparatus used for the manufacturing processes of the semiconductor device has a low decomposition ratio of 20%, for which reason the majority of the introduced PFC gas is discharged as the unreacted PFC gas. As described above, since the unreacted PFC gas is chemically stable, it is difficult to adsorb the unreacted PFC gas onto the adsorbent, for which reason the unreacted PFC gas is directly discharged to the atmosphere without any treatment or any process. The discharge of the unreacted PFC gas raises the serious problem with providing a great deal of influence to the global atmospheric system. In order to prevent this problem, an additional apparatus is required for compulsory treatment of the unreacted PFC gas to avoid direct discharge of the unreacted PFC gas into the atmosphere.
In order to decompose the unreacted PFC gas, it has been proposed to practice a combustion decomposition method, wherein the unreacted PFC gas is heated up to a high temperature of 1000.degree. C.-1200.degree. C. It has been concerned that this heat treatment at such high temperature may raise another serious problem with safety.
In the above circumstances, it has been required to develop a novel method and apparatus for treatment of the unreacted PFC gas to avoid direct discharge thereof to an atmosphere with safety and without substantial influence to a global atmospheric system.